In Austrian Patent 356,882, and the corresponding U.S. Pat. No. 4,247,206, there is described a screw for a double-screw extruder in which the screw is of the type having a double-conical screw design where the outside diameter decreases toward the exit side at a steeper pitch than the core of the screw.
In that patent, the screw is defined by the formula m.times.(A-B)/B=(C-D)/D. In this formula, A represents the outside diameter of the screw at the barrel entrance, B represents the outside diameter of the screw at the barrel exit, C is the screw flight depth at the barrel entrance and D is the screw flight depth at the barrel exit. In this patent moreover, m is a number between 0.5 and 0.75 and represents the difference in the degree of convergencies of the outer diameter of the screw where the outer periphery of the flight lies along a conical surface of greater apex angle and the root of the screw whose root surface lies along a conical surface of a lesser apex angle.
In practice, m can range between 0.01 and 0.5 in another commercial design and there has been suggestion in the art that m should range between 0.75 and 1.0. All of these systems are double-conical screws where the cone of the root and the cone of the outer periphery have different convergencies or apex angles, but always with the convergency of the outer periphery greater than the convergency of the root cone.
Where m=0, the two cones have the same apex angles or tapers.
The prior art double-cone designs described above were developed to increase the material which could be admitted to the screws in the feed section of the barrel and thereby increase the throughput of the screws. As a consequence, however, there is a continuous compression of the thermoplastic composition or polymer as it passes from the inlet to the outlet. This can cause excessive pressure and shear, especially at the exit region and, as a result, melt control is lacking, there can be degradation of the polymer, there may be excessive backflow, and there may be discharge at any barrel vent which may be provided.
While these problems are largely overcome with parallel extruder designs because of the lack of continuous increase in the compression forces, it is difficult to build reliable parallel high-pressure counter-rotating extruder gearboxes below about 80 mm diameter. The upstream shafts are not sufficiently separated to provide for proper bearings for them. Conical designs, therefore are preferred for lower capacity extruders since the spacings of the shafts at the upstream end will be greater and consequently a more robust gearbox can be designed.